Reducing machines are machines used to grind, chip, shred, or otherwise mechanically break down larger pieces of material into smaller pieces of material. One common type of reducing machine is known as a horizontal grinder.
A horizontal grinder typically includes a horizontally oriented power infeed mechanism that forces material into contact with a rotating grinding drum at the side of the drum. This configuration allows the material to be introduced to the drum at one side while also allowing material to be ejected through a screen, onto a discharge conveyor, at the bottom side and the opposite side of the drum from where the drum receives the material. Further the configuration allows any un-processed material at the top of the drum to be directed back to the infeed area. Specifically, some horizontal grinders have a rotating drum that operates at speeds greater than 500 RPM, and often in excess of 1000 RPM. The speed at which the rotating drum operates affects both the size reduction characteristics of the drum and the characteristics surrounding how material is fed to the rotating drum.
Many different types of material are processed with horizontal grinders. Some materials can be difficult to feed including various forms of wood-based material such as tree trunks, tree branches, logs, root balls. Woody materials, and many other materials as-well, are resilient, do not shear easily, and tend to self-feed. Self-feeding is not desirable as it can result in inconsistent loads on the drum, inconsistent productivity, and inconsistent quality of the ground product. The self-feeding tendency is affected by where the material is presented to the drum. Infeed systems that present material towards the bottom of the drum, where the material is presented nearly tangent to the path of travel of the teeth of the drum, tend to have frequent self-feeding. For example, U.S. Pat. No. 6,227,469 illustrates a horizontal grinder that presents material towards the bottom. This patent describes a solution for the self-feeding tendency associated with that configuration. In another example, U.S. Patent Publication No. 2005/0253003 discloses presenting material more toward the center-line of the drum. When material is fed toward the bottom of the drum, larger material can be introduced as compared to when material is fed closer to the centerline of the drum. This is due to the size of the infeed opening, which is the vertical distance between an in-feed mechanism and the exposed top of the drum, is larger. However, while introducing material toward the lower part of the drum allows for larger material to be processed, raising the point at which material is fed to the drum reduces the tendency for self-feeding.
After material is presented to the drum, which is mounted in a mill box, it is contacted by teeth carried by the grinding drum. Portions of the material are forced past a fixed shear edge defined by an anvil of the horizontal grinder. Upon passing the fixed shear edge of the anvil, the material enters the mill box defined at least in part by a screen that extends around a portion of the grinding drum and a mill housing that extends around, in close proximity to, a portion of the grinding drum. Within the mill box, the material is further reduced by the teeth carried by the grinding drum and interacting with the screen. Once the material within the mill box is reduced to a certain particle size, the material is discharged through the screen. Upon passing through the screen, the reduced material is typically deposited on a discharge conveyor that carries the reduced material to a collection location. An example horizontal grinder is disclosed in U.S. Pat. Nos. 7,971,818 and 7,441,719 which are hereby incorporated by reference in their entirety. The mill box also includes a top cover that directs material not passing through the screen back to the infeed. This overall arrangement results in an infeed opening defined at the bottom by the powered infeed mechanism and at the top by the mill lid that defines the exposed top of the drum.
A need exists for a horizontal grinder arranged to maximize the infeed opening, while limiting the self-feeding tendency.